Heretofore, in a safety system for closing a gate valve in a main flowline when the pressure in the flowline is outside a predetermined range, such as shown in U.S. Pat. No. 3,943,974 dated Mar. 16, 1976, a separate manual control valve has been utilized for resetting the pressure system after the fluid actuator for the gate valve has been vented to atmosphere. The manual control valve for resetting the fluid supply is gripped manually and moved to a position against a spring bias to supply fluid to the actuator for reopening the gate valve. Upon a build up of pilot line pressure in the high-low pressure sensors to the desired pressure range, a releasable detent holding the valve against the spring bias is released for placing the system in normal operating position.
A combined high-low pressure sensor has been provided heretofore with an automatic reset mechanism as shown in U.S. Pat. No. 4,116,215 dated Sept. 26, 1978. The shut-off valve is closed when the main flowline pressure is outside the predetermined operating range, and upon return of the flowline pressure to the normal or predetermined range, the shut-off valve is opened automatically without any manual reset mechanism.